In the prior art, it is known to deposit material onto a sample via ion beam induced deposition (IBID), typically performed in a focused ion beam (FIB) instrument, and electron beam induced deposition (EBID), usually performed in a scanning electron microscope (SEM) instrument. According to known methods, a sample is placed in an evacuable specimen chamber of a charged particle beam apparatus—typically either a FIB system or a SEM system. The charged particle (or other) beam is applied to the sample surface in the presence of a deposition gas, often referred to as a precursor gas. A layer of the precursor gas adsorbs to the surface of the sample. The thickness of the layer is governed by the balance of adsorption and desorption of the gas molecules on the sample surface, which in turn depends on, for example, the partial gas pressure, the substrate temperature, and the sticking coefficient. The thickness of the resultant layer can vary according to the application.
Material deposition can be performed with a variety of different gas precursors depending on the application. For example, tungsten hexacarbonyl (W(CO)6) gas may be used to deposit tungsten and naphthalene gas may be used to deposit carbon. A precursor gas of TEOS, TMCTS, or HMCHS gas in combination with an oxidizer such as H2O or O2 may be used to deposit silicon oxide (SiOx). For deposition of platinum (Pt), a (methylcyclopentadienyl) trimethyl platinum gas may be used.
The material depositions obtained from these different precursors have different properties. For example, IBID Pt material deposited using (methylcyclopentadienyl) trimethyl platinum precursor tend to be “softer.” That is, such softer materials are more susceptible to subsequent ion beam sputtering than “harder” IBID carbon or tungsten layers obtained with naphthalene or W(CO)6, respectively. Silicon oxide layers using a precursor of TEOS, TMCTS, or HMCHS gas in combination an oxidizer such as H2O or O2 tend to be more of a “medium” hardness. The relative “hardness” or “softness” of the material is dependent on the angle of incidence of the beam. In some material pairs the “harder” material becomes softer at a differing angle of incidence. Other differences exist as well. For example, when platinum films are used as sacrificial caps prior to FIB cross-sectioning (such as occurs in TEM preparation), the soft nature of the film tends to result in smooth cross-sectional cut face. By contrast, carbon films are extremely hard and tend to produce artifacts on the cut face known as “curtaining.” In addition to hardness properties the growth rates of these different deposition precursors may also be a significant factor for various applications.
The following are examples of gas precursors of various classes. For example, class C etchants may include oxygen (O2), nitrous oxide (N2O), and water. Metal etchants may include iodine (I2), bromine (Br2), chlorine (Cl2), xenon difluoride (XeF2), and nitrogen dioxide (NO2). Dielectric etchants may include xenon difluoride (XeF2), nitrogen trifluoride (NF3), trifluoroacetamide (TFA), and trifluoroacetic acid (TFAA). Metal deposition precursor gases may include (methylcyclopentadienyl) trimethyl platinum, tetrakis (triphenylphosphine) platinum (0), what is (0) tungsten hexacarbonyl (W(CO)6), tungsten hexafluoride (WF6), molybdenum hexacarbonyl (Mo(CO)6), dimethyl (acetylacetonate) gold (III), tetraethylorsosilicate (TEOS), and tetraethylorsosilicate (TEOS) plus water (H2O). Dielectric deposition precursors may include tetraethylorsosilicate (TEOS), tetraethylorsosilicate (TEOS) plus water (H2O), hexamethylcyclohexacyloxane ((HMCHS)+O2), and tetramethylcyclotetrasiloxane ((TMCTS)+O2). Carbon deposition precursors may include naphthalene and dodecane (C12H26), and planar delayering agents may include methylnitroacetate. Although these are many examples of available gas precursors many others exist and are available for use.
Beam induced deposition is used in a wide variety of applications for depositing a material onto a target surface of a sample such as a semiconductor wafer. The materials are deposited for a variety of reasons such as to form thin-film surfaces, electrical connections, protective coatings for semiconductor feature characterization and analysis, and capping material for milling high-aspect ratio structures (such as vias). However, when there is a significant difference between the hardness of the sample and the deposited capping material, it can be difficult to obtain the desired structure, shape, and surface characteristics of a prepared sample. For example, it can be difficult to control sloped surfaces in the formation of milled structures, because the differential sputter rates of the materials can cause slope changes at the interface between the materials. Additionally, artifacts can occur on FIB milled surfaces during a cross-sectioning process for preparing samples for feature characterization and analysis.
Techniques using FIB systems are known for preparing ultra-thin samples for feature characterization and analysis in which it is important to minimize the occurrence of surface artifacts introduced during the milling process.
As semiconductor geometries continue to shrink, manufacturers increasingly rely on transmission electron microscopes (TEMs) for monitoring the manufacturing process, analyzing defects, and investigating interface layer morphology. Transmission electron microscopes allow observers to see features having sizes on the order of nanometers. In contrast to scanning electron microscopes (SEMs), which only image the surface of a material, TEMs also allow analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are detected to form an image of the sample. A scanning transmission electron microscope (STEM) combines the principles of a TEM and SEM and can be performed on either instrument. The STEM technique scans a very finely focused beam of electrons across a sample in a raster pattern. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel through the sample and exit on the opposite side.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. The term “TEM” as used herein refers to a TEM or a STEM and references to preparing a sample for a TEM are understood to also include preparing a sample for viewing on a STEM. The term “STEM” as used herein also refers to both TEM and STEM.
There are several methods for preparing a thin sample for viewing with a TEM or STEM. Some methods entail extracting a sample without destroying the entire material from which the sample is extracted. Other methods require destroying the material to extract the sample. Some methods provide extraction of a thin sample referred to as a lamella. The lamella may require thinning before TEM or STEM viewing.
Lamella samples for TEM viewing are typically less than 100 nm thick, but for some applications samples must be considerably thinner. With advanced semiconductor manufacturing processes at design nodes of 30 nm and below, the sample needs to be less than 20 nm in thickness in order to avoid overlap among small scale structures. Some applications, such as analysis of next-node semiconductor devices, require lamellae having a thickness of 15 nm or less to isolate specific devices of interest. Current methods of thinning lamellae are difficult and not robust. Thickness variations in the sample result in sample bending or bowing, overmilling, or other catastrophic defects that may destroy the lamella. For such thin samples, preparation is a critical step in TEM analysis that significantly determines the quality of structural characterization and analysis of the smallest and most critical structures.
It is known to provide a protective layer deposited over the desired lamella location before thinning to protect the region of interest on the sample from exposure to the ion beam and to prevent bending or bowing. In one commonly used preparation technique as seen in FIGS. 1-3, a protective layer 22 of a material such as tungsten, carbon, or platinum is first deposited over the area of interest on a top surface 23 of a sample body as shown in FIG. 1 using electron beam or ion beam deposition. Next, as shown in FIGS. 2 and 3, a focused ion beam using a high beam current with a correspondingly large beam size is used to mill large amounts of material away from the front and back portion of the region of interest. The remaining material between the two milled areas 24 and 25 forming a thin vertical sample section 26 that includes an area of interest. Typically, the area of interest is contained in the top 200-300 nm below the sample surface. The area 25 milled on the back side of the region of interest is shown smaller that the front area 24. The smaller milled area 25 is basically to save time, but also prevents the finished sample from falling over into larger milled area 24 making it difficult to remove the sample section 26 from the sample body. Sample section 26 may then be cut away from the sample body using a focused ion beam and then lifted out using, for example, a micromanipulator, in a well-known manner. The sample section 26 is then typically transferred to a TEM grid and thinned. The sample section 26 may then be analyzed using a TEM or other analytical tools.
Significant problems occur in the preparation of ultra-thin (<30 nm thick) TEM samples. For example, a protective layer of platinum over the area of interest is too soft and often fails during lamella thinning, becoming completely consumed by peripheral erosion from the ion beam tails before the lamella thinning is complete. Layers of harder materials may resist erosion better than softer materials, but may cause undesirable artifacts on the cross-sectional face of the lamella.
FIGS. 4 and 5 show examples of problems in the preparation of ultra-thin samples. As seen in FIG. 4, a cross-section of a lamella sample 30 is shown prepared with a soft Pt cap 32 deposited on a hard diamond substrate 34. When the lamella 30 is prepared by thinning to the required thickness dimension, this hardness mismatch results in faster erosion of the softer Pt cap 32 than the harder diamond substrate 34. This combination would prevent the user from thinning the lamella as much as desired, because the protective cap 32 would eventually be completely consumed before the substrate 34 was adequately thinned. Conversely, as seen in FIG. 5, a cross-section of a lamella sample 36 is shown prepared with a hard carbon cap 38 placed on a soft copper substrate 40. In this example, undercutting 42 may be observed because the softer substrate 40 is consumed faster than the harder cap 38. This can lead to premature failure of the lamella, as well as cross-sectioning surface artifacts, such as “curtaining.”
Curtaining is an artifact that causes the surface of a sample to be rippled or uneven. Curtaining may arise for a variety of reasons. If the sample is non-homogeneous, consisting of different materials with different sputter rates, then the harder materials may form resistant areas that project slightly from the cross-sectional face. These projections shield regions below them, leading to vertical streaks that propogate downwards. FIG. 6 shows a sample 44 having a silicon substrate with a tungsten protective layer exhibiting curtaining. The “curtains” arise because the tungsten is harder or more resistant to sputtering from the ion beam than the silicon substrate. This leads to features that protrude slightly from the cross-sectional face of the substrate. The harder, overhanging tungsten basically shields the substrate directly below it leaving vertical projections of the tungsten. Alternatively, some hard capping materials form ripple-like or rectilinear patterns when exposed to the ion beam, even though the capping material is itself internally homogenious. FIG. 7 shows a sample 46 having a silicon substrate with a carbon protective layer exhibiting curtaining. When a cross-sectional mill is performed, the carbon layer material gradually assumes a highly textured surface. Thus, the topography of the carbon layer leads to curtaining in this example. Top-down thinning of a sample having these types of structural or density variations will cause vertical ridges or variations to propagate from the denser materials (i.e., metal lines) near the top of the sample (the top being defined as closest to the ion beam source) down the face of the cross-section, running in a direction parallel to the ion beam direction. Curtaining is most often observed in semiconductor materials where multiple patterned layers of materials having a low sputtering yield blocks a faster sputtering yield material. Curtaining may also be observed in materials exhibiting different topographic regions where changes in sputtering yields vary with the milling incident angle. Samples with voids also induce curtains. Curtaining artifacts reduce the quality of the TEM imaging and limit the minimal useful specimen thickness.
Another type of artifact is referred to as a “golf tee.” For example, a layer of either tungsten or carbon on top of a region of interest on a sample, which is typically a material such as silicon. The capping material and the silicon substrate have different “hardnesses,” (resistance to sputtering from the ion beam) resulting in a top-to-bottom thickness variation called a “golf-tee”, wherein the sample is thicker at the top and narrows to a thinner dimension so that the sample has a “golf tee” profile when observed in a Y-section. Since the region of interest is usually contained near the top surface of the lamella the thicker dimension can obscure the region of interest and cause a less than desirable sample for TEM viewing.
An example of a “golf-tee” effect can be seen in FIG. 8, which shows a TEM sample 50 with an ion beam induced deposition (IBID) tungsten protective layer 52 located on the top surface of sample 50. In this example, after thinning the sample 50 is 44 nm wide directly under protective layer 52 and narrows to 25 nm wide at 150 nm below protective layer 52. This thickness variation is the result of the differential etch rate between the silicon substrate and the tungsten protective layer. Tungsten is a harder, denser material than silicon and has a significantly lower etch rate, which causes the tungsten protective layer 52 to be wider than the lamella body. Typically, the region of interest is located in the general area where the “golf-tee” occurs obscuring or interfering with the region of interest for TEM viewing.
What is needed is an improved method of material deposition to obtain a controlled work piece surface that is free of surface artifacts and slope changes.